As a mask blank for a halftone-type phase shift mask, there is known a mask blank having the structure in which a semitransparent phase shift film made of a metal silicide-based material, a chromium film (light shielding film) made of a chromium-based compound, and an etching mask film (hard mask film) made of an inorganic-based material are laminated from a substrate side. When a phase shift mask is to be produced using such mask blank, first, an etching mask film is patterned by dry etching using a fluorine-based gas and using as a mask a resist pattern which is formed on a front surface of the mask blank. Then, a light shielding film is patterned by dry etching using a mixture gas of chlorine and oxygen and using the etching mask film as a mask, and further, a phase shift film is patterned by dry etching using a fluorine-based gas and using the pattern of the light shielding film as a mask (see WO-A-2004/090635 (Patent Document 1)).
Meanwhile, in order to increase an etching rate with optical characteristics of the light shielding film of the chromium-based compound kept unchanged, proposal has been made about the structure in which the light shielding film is made of a chromium-based material containing tin. In this case, when an upper layer of the light shielding film is a layer containing no tin or having a low content ratio of tin, and a lower layer thereof is a layer having a high content ratio of tin, only an etching rate of the lower layer (on the substrate side) may be improved over an etching rate of the upper layer (on a front surface side), and hence over-etching time may be set short. On the other hand, when the content ratio of tin on the substrate side is set low, an end point of etching may be detected more easily by monitoring chromium during the dry etching (see JP-A-2013-238776 (Patent Document 2)).